The E. C. Thompson U.S. Pat. No. 4,112,147 discloses a method of manufacturing a monolithic crystal filter having a solid electrode on one side of a crystal wafer, and a pair of split electrodes on an opposite side of the wafer, to define a pair of resonators. In that patent the electrodes are formed in a base plating operation to produce a filter having an initial resonator coupling frequency which is wider or greater than a final desired resonator coupling frequency and which has an initial midband frequency greater than a final desired midband frequency. The filter then is mounted in a vapor deposition apparatus and additional electrode material is uniformly plated on the pair of split electrodes to decrease or narrow the resonator coupling frequency to a value which is essentially equal to the final desired resonator coupling frequency. The filter next is inverted in the vapor deposition apparatus and additional electrode material is uniformly plated on the solid electrode to establish the midband frequency of the filter at a value slightly above the final value desired. When the electrodes are plated in this sequence, the resonator coupling frequency decreases substantially during the additional plating of the split electrodes, but remains essentially constant, within tolerance limits, during the plating of the solid electrode, whereby further adjustment of the resonator coupling frequency is not necessary.
After the monolithic crystal filter has been formed in the E. C. Thompson U.S. Pat. No. 4,112,147 as above-described, the filter is attached or "floated" to a metal header with the split electrodes facing outward. The filter then is fine-tuned to the desired final midband frequency by plating additional electrode material onto the split electrodes to balance the resonators at the final midband frequency. This procedure is disadvantageous since the additional plating on the split electrodes further reduces the resonator coupling frequency of the filter, sometimes to the point where the value of the resonator coupling frequency falls outside tolerance limits. The filter then must be subjected to additional processing, or may have to be discarded.
Accordingly, a primary purpose of this invention is to provide a new and improved method of fine-tuning a monolithic crystal filter without causing any significant change in the resonator coupling frequency of the filter.